Breakdown of hydrodynamics below four dimensions in a fracton fluid

Hydrodynamics is a universal effective theory that describes the thermalization of chaotic many-body systems, and depends only on the symmetries of the underlying theory. Although the Navier–Stokes equations can describe classical liquids and gases, quantum fluids of ultracold atoms or quark–gluon plasma, they cannot yet describe the phases of matter where particle motion is kinematically constrained. Here we present the nonlinear fluctuating hydrodynamics of models with simultaneous charge/mass, dipole/centre of mass and momentum conservation. This hydrodynamic effective theory is unstable below four spatial dimensions: dipole-conserving fluids at rest are unstable to fluctuations, which drive the system to a dynamical universality class with qualitatively distinct features from conventional fluids. In one spatial dimension, our construction is reminiscent of the well-established renormalization group flow of the stochastic Navier–Stokes equations; however, the fixed point we find possesses subdiffusive scaling rather than the superdiffusive scaling of the Kardar–Parisi–Zhang universality class. We numerically simulate many-body classical dynamics in one- and two-dimensional models with dipole and momentum conservation, and find evidence for the predicted breakdown of hydrodynamics. Our theory provides a controlled example of how kinematic constraints lead to a rich landscape of dynamical universality classes in high-dimensional models.